Due to a user reporting that this was still occurring; If you experience this issue, please ensure that you are not using a "Texture" asset instead of a "Material" with the "User Interface" domain for your set up.
When playing a video (in UMG or as a material) it will appear black on Android devices. However, the sound from the video plays as expected.
Result: The Video appears black, however the sound can be heard from the video
Expected: The Video would play and appear as it does in the editor
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-52648 in the post.